1. Field of the Invention
This invention relates to a process for the continuous purification of a liquid using ion exchange resins. In one aspect, this invention relates to the continuous purification of sugar syrups produced in the corn wet-milling industry.
2. Background Information
U.S Pat. No. 2,815,322 describes a method for continuous liquid purification utilizing a moving bed ion exchange system. In this method, an ion exchange resin is physically moved from an exhaustion zone to a regeneration zone in a continuous loop contactor.
U.S Pat. No. 2,985,589 describes a simulated moving bed purification system utilizing a chromatographic separation to separate two or more components from each other.
U.S Pat. No. 3,642,616 describes a continuous purification system utilizing a series of ion exchange vessels to provide soft water.
U.S. Pat. No. 4,522,726 discloses a series of ion exchange compartments mounted on a rotatable carousel with the inlets and outlets of the compartments connected to central rotating multiport valves.
The corn wet-milling industry commonly utilizes pairs of ion exchange vessels, connected in series, to provide a means of purifying various sugar syrups, including, but not limited to corn syrup, dextrose and fructose. The syrups can have varying degrees of impurities at different concentrations depending on where the syrup is in the wet-milling process. A pair of vessels, one containing cation exchange resin and one containing anion exchange resin are operated in series to provide a primary ion exchange system. The effluent from this pair is then fed to an identical pair of cation-anion vessels to provide a secondary, or polishing, ion exchange zone. A third pair of identical cation-anion vessels is kept off-line while they are being regenerated with acid and caustic solutions, respectively. Regeneration is completed by first sweetening-off the exhausted beds to displace the syrup. The first portion of the sweeten-off volume is sent forward as product. After a set volume is sent forward to product, the remaining volume is collected in a sweetwater surge tank. This large sweetwater volume is evaporated and then sent back to feed. Sweeten-off is followed by backwashing of the resins to remove fines. The resin beds are then eluted with dilute acid or caustic and rinsed with water to remove excess chemical. After rinse, the resin beds are put in stand-by until the next cycle begins. The resins can be in stand-by up to 75% of the service cycle time. When the primary pair becomes exhausted, they are taken off-line to be regenerated as outlined above. The pair, which was previously in the secondary, or polishing, position, now becomes the primary set of ion exchangers. The pair that has just been regenerated now is put into service at the secondary, or polishing, position. Before the freshly regenerated resin beds are placed in service, they are sweetened-on. In sweetening on, a first set volume of effluent is sent to drain. A second set volume is then sent to the sweetwater surge tank before the resin beds are ready to be placed in the secondary position. As is the case during sweetening-off, the collected sweeten-on sweetwater is evaporated and sent back to feed. The switching from primary to secondary to polishing positions is done by opening and closing a series of valves on inlet and outlet manifolds to the ion exchangers. This method of operating assures that the most freshly regenerated resin bed will be in the polishing position, assuring the highest quality of purified product. It also assures that the primary set of exchangers will be utilized to the maximum extent possible, thus maximizing the overall economics of the process. Typically, as the ion exchange resins age they require a longer rinse volume due to a gradual buildup of foulants, which inhibit the rinse kinetics. To reduce the rinse volume on these aged resins, a recirculation rinse mode can be employed during the regeneration rinse which directs the anion rinse stream, through a pump, and back into the cation and again through the anion. A cross regeneration mode may also be employed to remove proteinaceous materials from the resin beds. Cross regeneration is the method of applying an anion regenerant to a cation exchanger or a cation regenerant to an anion exchanger. This is then followed by a standard regeneration using dilute acid on the cation resin and dilute caustic on the anion resin using dosages to ensure complete conversion. Periodic cross regeneration of the cation exchanger with caustic soda, in a corn syrup demineralization application, is common in order to remove proteinaceous material from the resin. These six-bed ion exchange systems generally employ resin bed depths of about 8 feet and are typically run at linear flow rates of about 2 gallons per minute per square foot of pressure vessel area. Higher flows are limited by resin pressure drop in the deep beds. Because the ion exchange resin is not being moved through a series of vessels, resin attrition is kept to a minimum. Maintenance and downtime are minimized through the use of discrete valves as opposed to utilizing multiport valves. Although common in the industry, six-bed ion exchange systems have a number of drawbacks, including:
1. The systems have a large footprint, taking up large amounts of plant space.
2. The relatively deep resin beds require running at reduced flow rates due to pressure drop limitations of the resin bed.
3. They have intermittent, high flow regenerant discharges to waste, which usually require high-capacity surge tanks.
4. They require high-capacity sweetwater storage tanks, which are used to contain the low concentration sugar solutions, which are produced when rinsing the resins prior to regeneration and when pumping the feed syrup into a freshly regenerated ion exchanger.
5. The large amounts of sweetwater cause an intermittent flow of purified product, necessitating more surge/storage tanks for the product.
6. Relatively high backwash flow rates, rinse volumes and regenerant chemical dosages are required.
7. Large inventories of ion exchange resins, relative to the amount of product produced, are required.
There is a need for a liquid purification system which would have a smaller footprint, less ion exchange resin inventory, lower sweetwater volumes, more constant waste flow rates and relatively constant product flow rate, as compared to conventional purification systems. Such a liquid purification system would have lower regenerant flow rates, be operable at higher linear velocity flow rates without causing any adverse effects on product quality or causing any undue pressure drop across the ion exchange resin bed.
It is an object of the invention to provide a liquid purification system with a smaller footprint than a conventional liquid purification system of similar capacity.
It is a further object of the invention to provide a liquid purification system, which has a reduced volume and more constant wastewater flow rate than a conventional liquid purification system of similar capacity.
It is still a further object of the invention to provide a liquid purification system, which has less ion exchange resin inventory than a conventional liquid purification system of similar capacity.
It is still a further object of the invention to provide a liquid purification system with a more consistent product flow rate than a conventional liquid purification system of similar capacity.
It is still a further object of the invention to provide a liquid purification system which operates at a higher linear velocity flow rate than a conventional liquid purification system without causing degradation of product quality or undue stress to the resin bed due to excessive pressure drop.
It is still a further object of the invention to provide a liquid purification system, which has lower water requirements and higher chemical efficiencies than conventional liquid purification systems of similar capacity without any adverse effects on product quality.
It is yet a further object of the invention to provide a means to recirculate the rinse water as necessitated by resin aging.
It is a further object of the invention to provide a means to cross-regenerate the ion exchange resin in order to remove proteinaceous foulants.
These and other objects of the invention will be fully understood from the following detailed description of the invention and from the drawings appended to this application.
The method of the present invention has met the above needs.
The invention includes an operating method for the continuous purification of a liquid utilizing either single or dual ion exchange vessels, each vessel being physically divided into a discrete plurality of ion exchange zones, each zone having an inlet and an outlet. A valving means at the inlet and outlet of each ion exchange zone provides communication between adjacent ion exchange zones as well as to feed, product, sweetwater, rinse, recirculation rinse and regenerant chemical streams. In one aspect of this invention a first vessel contains a discrete plurality of cation exchange zones and a second vessel contains a discrete plurality of anion exchange zones. In another aspect of this invention, a single vessel contains a plurality of discrete cation ion exchange zones and a plurality of discrete anion ion exchange zones. In yet another aspect of this invention, a single ion exchange zone is contained in a single vessel with a plurality of discrete vessels comprising the ion exchange system.